Eomaia scansoria (Chinese Academy of Geological Sciences (CAGS) 01-IG-1a,
b; holotype). a, Fur halo preserved around the skeleton (01-IG-1a, many
structures not represented on this slab are preserved on the counter-part
01-IG-1b, not illustrated). b, Identification of major skeletal
structures of Eomaia. c, Reconstruction of Eomaia as an
agile animal, capable of climbing on uneven substrates and branch walking.
Taken from ref (1)

This fossil was found in Lower Cretaceous
Yixian formation. It is extremely well preserved, and both part and
counter-part were recovered. It is a complete cranial and post-cranial
skeleton of a small (large mouse-sized) mammal that, dated at 125Myr,
represents the earliest eutherian (placental) animal fossil found. (The previous oldest,
Murtoilestes is dated at 120M years).

The new species is named Eomaia (Dawn
Mother in Greek) scansoria (climber in Latin).

The new fossil clearly
has some transitional features. In particular:

Epipubic bones extending
forwards from the pelvis (normally found in marsupials but not in placental
mammals, except in early transitionals). Reilly and White (4)
have recently identified a non-reproductive function for the epibubis in
primitive mammals. They show that the epipubic bone is part of a kinetic
linkage extending from thefemur, by way of the pectineus muscle, to
the epipubic bone, throughthe pyramidalis and rectus abdominis muscles on one side of theabdomen, and through the contralateral external and internal obliquemuscles to the vertebrae and ribs of the opposite side. This muscleseries is activated synchronously as the femur and
contralateralforelimb are retracted during the stance phase in
locomotion.The epipubic
bone acts as a lever that is retracted (depressed)to stiffen the
trunk between the diagonal limbs that support thebody during each
step. The epipubuc bone therefore has more of a function in locomotion than in reproduction.
The stiffening function
of the epipubic bone appears to be
theprimitive conditions for mammals.

A narrow pelvic outlet which
indicates that a short
gestation was followed by a parental nurture of the young suspended from the
abdomen after the fashion of modern marsupials.

There are many other
transitional mammalian features, such as 5 incisors in the upper jaw which is
diagnostic of metatherians (marsupials) although the molar to premolar
ratio (it has more pre-molars than molars) places it firmly amongst the eutherians (placentals).

Given
this mosaic of metatherian and eutherian features, along with many other derived and
primitive features, it is clear that Eomaia has transitional characteristics.

The animal seems to be specialised for climbing
and the
speculation is that this ability allowed placentals (and marsupials who also
have climbers in their early lineage) to out-compete the many sub-classes of
mammals that have become extinct (only three sub-classes survive today: placentals;
marsupials, like kangaroos etc; and monotremes like the duck billed platypus
- but the Cretaceous was a melting pot of mammalian sub-classes most of which do not
survive to today).

Some of the reasons that the fossil points to a climbing or tree-living habit
are:

1: Fore and hind feet of Eomaia show similar proportion and
curvature of the phalanges (the bones making up the fingers and
toes) to the grasping feet of extant arboreal mammals (opposum,
flying lemur and arboreal primates).

2: The
first joint of the forefoot digit is curved and it shows compelling
evidence of the attachment of a strong muscle that closes the
grasp around branches (absent in ground-dwellers)

3:
The length of the intermediate phalanx (second joint) of the forefoot as a percentage of the proximal (first joint) varies
according to the habit of the animal. It ranges from around 53% in fully terrestrial mammals (eg Metachirus) to 126% in fully arboreal
mammals (the flying lemur, Cynocephalus). Scansorial mammals have a lower ratio than
fully aboreal animals(eg, the tree shrew, Tupaia, has 61%). E scansoria is, at 79%, more arboreal
than scansorial.

4: On the hindfoot, digits (toes)
4 and 5 are longer than digits 1,2 and 3: this is a scansorial
pattern.

5: On the hindfoot, the bones of the digits 4
and 5 are longer than those of digit 3; In terrestrial mammals, the
bones of digits 2 and 3 are longer than those of digits 4 and
5

6: Turning to the claws, they have the curved shape
and show the insertion of strong flexor (grasping) muscles and are
very much likethe dormouse (Glis glis) and the tree shrew
(Tupaia).

7: Clear evidence of insertion of strong
climbing muscles in the shoulder blade

8:
The tail is twice as long as the rest of the spine, and the tail vertebrae are
elongated

9: The trapezoid and capitate
are small and in the same proportion to the hamate and trapezium
(all of these are wrist bones) as in extant climbing and tree
dwelling mammals

The palaeontological age of the fossil matches very
well with the molecular data which suggests a diversification of the major
clades within the placental sub-class at 110Myr. This age is estimated
by analysing the genetic divergence between the most distant extant placental
mammals according to a phylogenetic analysis.

The extraordinarily
well-preserved fossil includes a detailed fur halo.